The Immortal Six Hundred were 600 Confederate officers who were held prisoner by the Union Army in 1864–65. In the summer of 1863, the Confederacy passed a resolution stating all captured African-American soldiers and the officers of colored troops would not be returned. The resolution also allowed for any captured officer of colored troops to be executed and any captured African-American soldier be sold into slavery. The resolution caused a breakdown in the exchange of captured soldiers as the Union demanded all soldiers be treated equally. The Immortal Six Hundred were one group of officers who could not be exchanged.

In June 1864, the Confederate Army imprisoned five generals and forty-five Union Army officers in the city of Charleston, South Carolina, using them as human shields in an attempt to stop Union artillery from firing on the city. In retaliation, United States Secretary of War Edwin M. Stanton ordered fifty captured Confederate officers, of similar ranks, to be taken to Morris Island, South Carolina, at the entrance to Charleston Harbor. The Confederates were landed on Morris Island late in July of that year.

The Confederates had originally contended that Charleston should not be shelled. The correspondence between Major General John G. Foster, commanding the Federal Department of the South, and Major General Samuel Jones, commanding the Confederate Department of South Carolina, Georgia, and Florida, indicates the Confederates subsequently accepted the military nature of Charleston as a target. Soon the correspondence turned to an exchange of these high-ranking prisoners.

Instructions from the War Department reached Foster in late July, and he coordinated an exchange of the fifty prisoners on July 29. Exchange of the fifty officers actually took place on August 4, 1864. However, at that time Jones brought 600 additional prisoners to Charleston, in part to press for a larger prisoner exchange. In retaliation for the treatment of Federal prisoners, Foster asked for a like number of Confederate prisoners to be placed on Morris Island. These men became known in the South as the Immortal Six Hundred.

At one point General Foster planned an exchange of the six hundred, but General Ulysses S. Grant refused, following Order 252 which stated no exchanges would occur until the Confederacy agreed to treat both black and white prisoners of war equally. Grant wrote, "In no circumstances will he be allowed to make exchanges of prisoners of war."

The Confederate prisoners did not arrive on Morris Island until the first week of September 1864. During the first week of October 1864, Jones (under orders from Lieutenant General William J. Hardee) removed the Federal prisoners from Charleston. Foster removed the Confederate prisoners from Morris Island only after being informed officially of the Federal prisoners' status. At that time the Immortal 600 were moved to Fort Pulaski.

Three of the six hundred died from subsistence on starvation rations issued as retaliation for the conditions found by the Union at the Confederate prisons in Andersonville in Georgia and at Salisbury Prison in North Carolina.

Upon an outbreak of yellow fever in Charleston, the Union officers were removed from the city limits. In response the Union Army transferred the Immortal Six Hundred to Fort Pulaski outside of Savannah.